Value it
Look up the value of your car, based on its age, mileage and condition, in a used car price guide such as Parker's. Check this price against the ads in your local newspaper, Auto Trader or Exchange and Mart, as there are regional differences in pricing. Be honest with yourself about your car's condition - a good trick is to ask yourself, 'Would I pay that for this car?' Be realistic - you will not get the same price for your car as a dealer could get, nor will you recoup money spent on optional extras.
Clean it
Get cleaning, inside and out. Inside, you'll need to hoover the carpets, polish the dashboard, and empty all the detritus from door pockets, gloveboxes, ash-trays, cubby-holes and boot. And don't forget underneath the seats.
Clean and polish the exterior, including the wheels, windows and any chrome bits. If the paintwork's looking a bit tired and faded, give it a going-over with T-Cut or a similar paint enhancer. If you're hoping to get a value in the thousands for your car, then it's probably worth paying out £50 or so for a professional valet.
Tidy it up
Make sure oil, coolant, screenwash, brake fluid, power steering fluid etc are all topped up and relatively clean - it gives the impression that the car hasn't been neglected.
For the same reason, replace blown headlight bulbs, worn windscreen wipers or worn floor mats (non-brand mats are very cheap). Consider replacing bald tyres, having minor dents knocked out, and touching up little scratches and scrapes - all these devalue your car and give a buyer more ammunition to knock down your price. On older cars, it's worth stitching up tears in seat fabrics, sticking patches over cigarette burns in the upholstery and gluing up any flapping pieces of trim.
